Automatic frequency control



Oct. 7, 1969 J. STARK, JR

AUTOMATIC FREQUENCY CONTROL 2 Sheets-Sheet 1 Original Filed June 6. 1963Att ney United States Patent O 26,686 AUTOMATIC FREQUENCY CONTROL JohnStark, Jr., Indianapolis, Ind., assignor to Radio Corporation ofAmerica, a corporation of Delaware Original No. 3,264,408, dated Aug. 2,1966, Ser. No. 286,075, June 6, 1963. Application for reissue July 22,1968, Ser. No. 764,351

Int. Cl. H04n 3/16, 5/38 U.S. Cl. 178-7.3 6 Claims .Matter enclosed inheavy brackets [I appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates theadditions made by reissue.

ABSTRACT F THE DISCLOSURE A phase detector is described for automaticfrequency control of the horizontal oscillator in a television receiver.The circuit includes a pair of peak detectors arranged so that they areeectively in parallel with respect to incoming horizontal sync pulsesderived from the sync separator and in series opposition with respect topulses derived from the horizontal oscillator and output circuit. Whenthe two applied pulse trains are exactly in phase, the peak detectorsproduce outputs which cancel. When an out of phase condition occurs,incomplete cancellation results and a control voltage is applied to thehorizontal oscillator to shift its frequency. One of the peak detectorsnormally has an enhanced response to low frequency components in theapplied signals compared to that of the other peak detector.Accordingly, since it s desirable to balance out the overall response ofthe peak detector to these low frequency components, an impedance meansis added in the circuit to render the response characteristic of thepeak detectors more nearly equal. Equalizing the responses of the peakdetectors thereby e'ects a more complete cancellation of thesecomponents of the applied signals and the overall response of the phasedetector to low frequencies is attenuated.

The present invention relates to an automatic frequency control circuit.More particularly the present invention relates to a phase detectioncircuit for automatically controlling the frequency of the horizontaldeflection oscillator in a television receiver.

In television receivers, a local horizontal deflection oscillatorcontrols the repetition rate of the horizontal deflection circuit usedfor reproducing the televised image on the screen of the cathode raytube. This oscillator must be properly synchronized with the oscillatorat the TV transmitter which originally produced the televised image. Thetransmitted signal accordingly is a composite signal containing bothvideo information and horizontal and vertical synchronizing signals forthe purpose of synchronizing the horizontal and vertical deflectionoscillators at the television receiver. In this manner accuratereproduction of the televised image may :be obtained.

Phase detector circuits have been developed for comparing the time ofoccurrence of the horizontal synchronizing signal with the output of thehorizontal oscillator in the television receiver. With such circuits, anoutput voltage is obtained which is a function of the difference in thetime of occurrence between the output of the oscillator and thesynchronizing signal.

Reissued Oct. 7, 1969 "ice While these phase detector circuits aregenerally satisfactory for television viewing purposes there is room forimprovement. For example, when the television channel to which thereceiver is timed is changed, there may be momentary loss ofsynchronization before the control circuit starts to operate properly.Also, if there is loss of synchronization between the receiver and thetransmitter for any reason, the recovery time of conventional phasedetection circuits may not be as rapid as desired.

It is accordingly an object of the present invention to provide a newand improved automatic frequency control circuit which overcomes theabove disadvantages.

A second object of the present invention is to provide a new andimproved automatic frequency control for the horizontal oscillator of atelevision receiver.

Another object of the present invention is to provide a phase detectorcircuit for a television receiver which has improved frequency response.

Still another object of the present invention is to pro vide a new andimproved phase detector for the horizontal deilection oscillator of atelevision receiver which detector has a faster recovery time than ispresently available.

The present invention contemplates an automatic frequency controlarrangement including a pair of peak detector circuits; a first sourceof periodic signals occurring at a first frequency and having an outputapplied to the pair of peak detectors arranged in parallel; a secondsource of periodic signals occurring at substantially the firstfrequency and having an output applied to the pair of peak detectorcircuits arranged in series opposition, the pair of peak detectorcircuits thereby producing a control voltage which is a function of anyphase difference between the periodic signals of the first and secondsources; and means tending to balance the voltage developed by thedetector circuit. Control means are also included responsive to thecontrol voltage and connected to the second source of periodic signalsfor changing the rate of occurrence of the periodic signals of thesecond source in a direction to reduce the difference in frequency ofthe two periodic signals to zero. The balancing means not only tends tobalance the detector circuit voltages, but gives the circuit a fasterresponse, reduces ringing, and has other advantages.

The novel features which are considered as characteristie for theinvention are set forth in particular in the appended claims. Theinvention itself. however, both as to its construction and its method ofoperation together with additional objects and advantages thereof willbest be understood from the following description of specificembodiments when read in connection with the accompanying drawings inwhich:

FIGURE l is a schematic diagram partially in block form of a circuitincorporating the principles of the present invention;

FIGURES 2 and 3 are graphical representations of the wave shapesoccurring at different portions of the circuit for the purpose ofillustrating the advantageous operation thereof.

Referring to the drawings and more particularly to FIGURE 1 thecomposite signals transmitted by a television transmitter are receivedby an antenna 10 and applied to the radio frequency (RF) tuner and theintermediate frequency (IF) amplifier represented by the block 11. TheIF signals produced by the intermediate frequency amplifier of block llare applied to the video detector and video amplifier represented by theblock 12. At this stage of the television receiver, in accordance withconventional practice, the video information to be displayed on thescreen of the cathode ray tube and the synchronizing signals have beendemodulated from the RF carrier signal. The output signal from the videoamplifier of the block 12 is applied on conductor 13 to the cathode raytube 14 to be displayed on the screen thereof.

A second output signal obtained from the block 12 is the modulated soundsignal which is applied on conductor 16 to the sound channel 17 whereinit is demodulated and reproduced in synchronism with the displayedimage. The apparatus represented by the blocks 11, 12 and 17 may beconstructed in any one of several conventional configurations and acomplete illustration of these circuits is not provided in order toavoid unnecessarily complicating the drawings.

A third output signal from the block 12 is applied on conductor 18 tothe synchronizing signal separator 19. The synchronizing signalseparator 19 operates in a conventional manner to separate thehorizontal synchronizing signals from the vertical synchronizing signalsand from the video signals, all of which are contained in the originalsignals received by the antenna 10.

The vertical synchronizing signals are applied from the sync separator19 on conductor 21 to a vertical deliection circuit 22. The verticaldeliection circuit 22 produces a sawtooth current which is applied fromits output terminals 23 to the terminals V-V of the vertical detiectingcoils 24 of the cathode ray tube 14.

Also produced at the output of the sync separator 19 on a conductor 26are the horizontal synchronizing signals which are applied through acoupling capacitor 27 to the junction point 28 between diodes 29 and 31which forni a portion of the phase detection circuit. The diode 29 has aload resistor 32 and a compensating capacitor 30 connected in paralleltherewith while diode 31 is connected in parallel with a load resistor33. The junction between the anode of the diode 31 and the resistor 33is connected to a source of datum potential or ground for the televisionreceiver. l

The junction of the anode of the diode 29, the capacitor 30 and theresistor 32 is identified by the numeral 34. Junction point 34 isconnected to one terminal of a frequency response compensating resistor38, the other terminal of which is grounded, and to one terminal o f aload capacitor 37 and a resistor 39. The other terminal of capacitor 37is grounded while resistor 39 is connected to one terminal of, and formsan integrating network with a capacitor 41 the other terminal of whichis grounded.

The junction between resistor 39 and capacitor 41 1s connected to thecontrol electrode of a horizontal control tube 40. The control electrodeof tube 40 is connected to ground by the series combination of aresistor 42 and a capacitor 43. The cathode of tube 40 is connected toground by a bias resistor 44 and the anode thereof is connected by meansof anode resistors 45 and 46 to the positive terminal 47 of a source ofoperating potential. The junction between resistors 45 and 46 isbypassed to ground by capacitor 55. The anode of tube 40 is connected toground through biasing resistor 48 and to the movable tape of ahorizontal hold potentiometer 49. One end of potentiometer 49 isconnected to ground by means of resistor 51 while the other end ofpotentiometer 49 is connected to the control electrode of a horizontaloscillator tube 50 by means of a resistor 52.

The cathode of oscillator tube 50 is connected to ground by the tunedcircuit 53 made up of a variable inductor and a capacitor. The anode oftube 50 is coupled to the control electrode thereof by means of a tappedinductor S3 and capacitor 54 in series. The anode of tube 50 is alsoconnected to the positive terminal 47 of the source ot operatingpotential by means of anode resistors 56, 57 and 46 connected in seriesfrom the terminal 47 to the tap on tapped inductor 53. The periodicsignals from the oscillator tube 50 are applied from the inductor 53 tothe input of the block 58 representing the horizontal output circuit ofthe television receiver.

In accordance with conventional practice, the horizontal output circuit58 includes a horizontal output tube and an output transformer in whichis developed the sawtooth currents for the horizontal defiection coilsof the cathode ray tube. These currents are applied on output terminals59 to the terminals H-H of the horizontal defiecting coils 61 of thecathode ray tube 14. The horizontal output tube is alternately renderedconductive and nonconductive at each cycle of the oscillations producedby the oscillator 50` to provide the horizontal deiiection currents inaccordance with conven tional reaction scanning techniques.

The oscillatory output of the tube 50I is also fed back to the phasedetector circuit by means of a coupling capacitor 36 connected from thejunction between resistors 56 and 57 to the junction point 34. Also fedback to the phase detection circuit are sawtooth pulses produced by thehorizontal output circuit 58. These sawtooth pulses are connected byconductor 62 through coupling capacitor 35 to the junction point 34.

In operation, the phase detector circuit has two inputs, namely thejunction point 28 and the junction point 34. To the junction point 28are applied the horizontal synchronizing signals from the sync signalseparator 19 which may have the shape illustrated in FIGURE l by thewaveform 63. These represent a series of first periodic signalsoccurring at the horizontal synchronizing scanning rate as establishedby the transmitted composite signal.

To the junction point 34 are applied the outputs from the horizontaloscillator tube 50 and the horizontal output circuit 58. The waveformapplied to the junction point 34 as a series of second periodic signalsis represented by the waveform 64 in FIGURE l. Since the horizontaloscillator tube 50 is tuned to oscillate at subA stantially thehorizontal scanning frequency both waveforms 63 and 64 occur atsubstantially the same repetition rate or frequency.

The load for the phase detector circuit may be taken as the capacitor37, filters 39, 4l, and the input circuit of the horizontal control tube40 ignoring for the time being the resistor 38. The peak detectorcircuits represented respectively by diode 29 with its load resistor 32and diode 31 with its load resistor 33 appear in parallel to thehorizontal synchronizing signals 63 applied to the junction 28. Thesesame peak detector circuits appear in series to the signal 64 applied tothe junction 34. In accordance with known balanced phase detectiontechniques, a voltage is produced across the capacitor 37 which is afunction of the difference in phase between the horizontal synchronizingsignals applied to junction 28 and the signals applied to junction 34.Briefly, the peak detectors made up of diode 29 and resistor 32 anddiode 31 and resistor 33 produce average voltages across the respectiveresistors 32 and 33 ywhich are equal approximately to the peak of thenegative pulses of the horizontal synchronizing signals 63 superimposedin opposite polarities on the horizontal output signals 64, duringretrace time.

If the sawtooth pulses 64 applied to junction 34- are exactly in phasewith the horizontal synchronizing pulses 63, the sawtooth voltage ispassing through its A.C. axis in one direction, say negative going, inthis case, at the sarne instant that the horizontal synchronizing pulse,also negative going, is causing the diodes 29 and 31 to conduct.Therefore, during such inphase conditions there will be no D.C. voltagedeveloped across the rcsistors 32 and 33, Le., the respective voltagesacross these resistors is thereorectically balanced out. lf thehorizontal oscillator is operating at a lower frequency so that thefedbacl; sawtooth pulses 64 lag the horizontal synchronizing pulses 63 anegative voltage (with the `polarities indicated for the appliedsignals) will be developed across capacitor 37 and applied to thehorizontal control tube to speed up the oscillator 50. If the horizontaloscillator is operating at a higher frequency the sawtooth pulsesapplied to junction 34 will occur before the horizontal sync pulses andlead these pulses. Accordingly a positive voltage will be developed andapplied to the horizontal control tube to slow down the oscillator.Hence the oscillator tends to be stabilized at a synchronous frequencysubstantially in phase with the horizontal synchronizing voltage.

The conventional balanced phase detector, as its operation is thus fardescribed, may have several disadvantages, as illustrated by thewaveforms in FIGURES 2a and 3a, which disadvantages the circuit ofFIGURE l will substantially eliminate. The waveforms of FIG- URES 2a and3a are taken in the absence of resistor 38. In FIGURE 2 and moreparticularly in subFIGURE 2a, the prior art waveform 63 appearing at theoutput of sync signal separator 19 is illustrated on a much longer timescale than that of FIGURE 1, and therefore appears greatly compressed.Two vertical synchronizing intervals 71 are shown. It can be seen that,after the occurrence of each vertical synchronizing interval, a ringingeffect may be present evidenced by the damped oscillation indicated at72 in the envelope of the waveform 2a. Due to such ringing the phasedetection circuit may wander or hunt about the proper synchronizingfrequency for the horizontal oscillator. This may cause the beginning ofthe displayed video signal to also hunt, possibly producing a bending ofthe top portion of the reproduced image.

A second possible disadvantage of prior art circuits is shown in FIGURE3a. In this ligure the waveform 65 appearing across the load capacitor37 is illustrated on a time scale comparable to that of FIGURE 2. Theaverage value of the waveform 65 is applied to the control electrode ofthe horizontal control tube 4() for correction of any phase differencesbetween the output of the horizontal oscillator and the incominghorizontal synchronizing pulses. The dips 73 in the waveform 65 of FIG-URE 3a show that there is a corresponding change or dip in the averagevalue of the signal which occurs on those occasions when the horizontalsync of the television receiver is momentarily lost. This may occur whenthe receiver is switched from channel to channel or during the verticalretrace or due to interference, etc. This large change in the signalwhich is applied to the horizontal control tube may again contribute tohunting or loss of information in the portion of the reproduced imageoccurring during or immediately after such time interval.

Now consider the effect of adding the resistor 38 between the junctionpoint 34 and ground. The resistor 38 serves as a ground return for theanode of the diode 29. In the absence of the resistor 38, the diode 31which has a grounded anode has a much better response to low frequencycomponents in the horizontal synchronizing signal than the diode 29.This low frequency response may cause an error signal to be applied tothe horizontal control tube even though the horizontal oscillator may infact be properly synchronized with respect to the incoming horizontalsynchronizing signals.

The low frequency components which may be troublesome are generallypresent in the horizontal synchronizing signals occurring during thevertical sync and vertical retrace interval. By adding the ground returnresistor 38 for the anode of diode 29 both of the diodes respond to thelow frequency components more nearly in substantially the same manner sothat these low frequency components tend to be balanced out. Thisbalancing reduces any error signal that may be present due to thedifferences in the frequency response of the diodes 29 and 31.

The waveforms corresponding to those of FIGURES 2a and 3a, except thatresistor 38 has now been added to the circuit, are shown in FIGURES 2band 3b, respectively. From FIGURE 2b it can be seen that the presence ofthe ground return resistor 38 in the circuit substantially eliminatesthe ringing from portion 74 of the signal occurring immediately afterthe vertical blanking interval thus reducing the undesirable huntingthat may otherwise be present immediately following the vertical syncinterval.

Similarly, with the ground return resistor 38 present in the circuit,FIGURE 3b shows that changes in voltage due to loss of sync or otherinterference are reduced by the wave shaping effect of the resistor 38.The dips 76, it can be seen, are substantially smaller than the dips 73of FIGURE 3a. Thus, utilization of the resistor 38 provides quickerrecovery of the horizontal oscillator after the loss of sync that mayoccur for various reasons.

Thus, the output waveform of FIGURE 3b shows that the phase of theoscillator 50 signals now follows more closely the phase of thehorizontal synchronizing signals than without the resistor 38. Thevoltage produced across capacitor 37 is applied to the control electrodeof the horizontal control tube 40 by way of the filter networks made uprespectively of resistor 39 and capacitor 41 and resistor 42 andcapacitor 43.

The tube 40 effectively acts as a D.C amplifier, its output beingapplied to the control electrode of the oscillator tube 50 by way ofhorizontal hold potentiometer 49 and resistor 52. The frequency ofoscillator 50 is thereby tuned by this amplified voltage to exactly theproper synchronized frequency. The output signals from tube 50 areapplied to the horizontal output circuit represented by the block 58 forthe production of horizontal deflection waveforms, as explained above.

Without any intent to limit the scope of the present invention a circuitutilizing the following components has been found to have the beneficialeffect represented by the waveforms 2b and 3b:

Capacitor 27-47 Diodes 29, 31-GE Dual Selenium Capacitor 30--68 M.M.F.Resistors 32, 33-390,000 ohms each Capacitor 36-27 M.M.F. Capacitor37-820 M.M.F. Resistor 38-120,000 ohms Resistor 39-270,000 ohms As hasbeen explained above, the improved results obtained by the circuitillustrated in FIGURE l have been attributed to the improved response ofthe diode 29 to low frequency components that may be present in thehorizontal synchronizing signal. It is possible that the same resultsmay be achieved by otherwise equalizing the respective frequencyresponses of each of the diodes. It might be possible for example tobalance the respective low frequency responses of diodes 31 and 29 byinserting an impedance between the anode of diode 31 and ground. Sincethe detector is preferably solely phase sensitive, any modificationwhich balances the responses of each of the respective diodes isdesirable.

While the above invention has been described with respect toimprovements in an automatic frequency control circuit for use insynchronizing a horizontal oscillator in a television receiver, it isclear that the same principles may be applied to other frequency controlor phase detection circuits to achieve the benefits above described.

What is claimed is:

1. An automatic frequency control comprising, in cornbination,

a first and a second peak detector circuit',

a source of first periodic signals occurring at a rst frequency, saidfirst periodic signals being applied to said first and second peakdetector circuits arranged in parallel;

a source of periodic signals occurring at substantially said firstfrequency, said second periodic signals being applied to said peakdetector circuits connected in series opposition, a capacitive loadcircuit, said first and second peak detector circuits producing acontrol voltage across said load circuit when said first and secondperiodic signals are applied thereto, said control voltage beingproportional to any phase difference between said first and secondperiodic signals;

control means responsive to said control voltage and connected to saidsource of second periodic signals for changing the frequency of saidperiodic signals in a direction to reduce said control voltage tosubstantially zero; and

impedance means coupled across said load circuit for substantiallyreducing the combined response of said [first] peak detector [circuit]circuits to low frequency variations of the periodic signals supplied bysaid first source thereby tending to equalize the frequency response ofsaid peak detector circuits.

2. An automatic frequency control arrangement for television receiverscomprising, in combination,

a first peak detector circuit having a first parallel combination of adiode and a resistor',

a second peak detector circuit having a second parallel combination of adiode and a resistor, the anode of said diode of said second combinationbeing arranged substantially at electrical ground potential;

a source of horizontal synchronizing pulses occurring at the horizontalsynchronizing frequency of the television receiver, said signals beingapplied to said first and second peak detector circuits arranged inparallel;

a source of second periodic signals occurring substantially at thehorizontal synchronizing frequency of the television receiver, saidsecond periodic signals being applied to said first and second peakdetector circuits arranged in series;

a capacitive load circuit coupled across the series combination of saidfirst and second peak detector cir cuits;

said peak detector circuits thereby producing a control voltage acrosssaid load circuit varying as a function of the phase difference betweensaid horizontal synchronizing pulses and said second periodic signals;

control means responsive to said control voltage for changing thefrequency of said second periodic signals in a direction to reduce saidphase difference substantially to zero; and

impedance means coupled between the anode of the diode of said firstparallel combination and ground potential across said capacitive loadcircuit for substantially reducing the combined response of said [first]peak detector circuits to low frequency variations of the amplitude ofsaid horizontal synchronizing pulses thereby tending to equalize thefrequency response of said peak detector circuits.

3. An automatic frequency control arrangement for television receiverscomprising, in combination,

a rst peak detector circuit having a first parallel combination of adiode and a resistor;

a second peak detector circuit having a second parallel combination of adiode and a resistor, the anode of said diode of said second combinationbeing arranged substantially at electrical ground potential;

a source of horizontal synchronizing pulses occurring at the horizontalsynchronizing frequency of the television receiver, said signals beingapplied to said first and second peak detector circuits in parallel;

a source of second periodic signals occurring substantially at thehorizontal synchronizing frequency of the television receiver, saidsecond periodic signals being applied to said first and second peakdetector circuits in series;

a capacitive load circuit coupled across the series cornbination of saidfirst and second peak detector circuits;

said peak detector circuits thereby producing a control voltage acrosssaid load circuit varying as a function of the phase difference betweensaid horizontal synchronizing pulses and said second periodic signals;

control means responsive to said control voltage for changing thefrequency of said second periodic signals in a direction to reduce saidphase difference; and

impedance means including at least one resistor for returning the anodeof the diode of said first parallel combination to ground potential forsubstantially reducing the combined response of said [first] peakdetector circuits to low frequency variations of the amplitude of saidhorizontal synchronizing pulses thereby tending to equalize thefrequency response of said peak detector circuits.

4. An automatic frequency control arrangement for television receiverscomprising, in combination,

a first peak detector circuit having a first parallel combination of adiode and a resistor;

a second peak detector circuit having a second parallel combination of adiode and a resistor, the anode of said diode of said second combinationbeing arranged substantially at electrical ground potential;

a source of horizontal synchronizing pulses occurring at the horizontalsynchronizing frequency of the television receiver, said signals beingapplied to said first and second peak detector circuits in parallel;

a source of second periodic signals occurring substantially at thehorizontal synchronizing frequency of the television receiver, saidsecond periodic signals being applied to said first and second peakdetector circuits in series;

a capacitive load circuit coupled across the series combination of saidfirst and second peak detector circuits;

said peak detector circuits thereby producing a control voltage acrosssaid load circuit varying as a function of the phase difference betweensaid horizontal synchronizing pulses and said second periodic signals;

control means responsive to said control voltage for changing thefrequency of said second periodic signals in a direction to reduce saidphase difference; and

impedance means including a compensating resistor for returning theanode of the diode of said first parallel combination to groundpotential, the value of resistance of said compensating resistor beingsubstantially smaller than the value of resistance of either of saidresistors in said first and second parallel combinations to [reduce]increase substantially the low frequency response of said first peakdetector without substantially affecting the low frequency response ofsaid second peak detector.

5. An automatic frequency control arrangement for television receiverscomprising a capacitive load circuit;

first and second rectifiers coupled in series opposition across saidload circuit;

first and second resistors coupled respectively across said first andsecond rectifiers;

a first source of periodic signals of predetermined frequency;

means including said load circuit for connecting said rectifiers inparallel across said first source;

a second source of periodic signals of substantially the samepredetermined frequency;

means for connecting said second source to said rectifiers in seriesopposition;

and impedance means coupled across said load circuit for substantiallyreducing the Combined response of said [first rectifier] recliers to lowfrequency varia.-

9 10 tions in amplitude of the periodic signals supplied UNITED STATESPATENTS by Said fllSt SOUI'CC thereby tending t0 equalize the 2'6696552/1954 Gruen 328 134 frequency response 0f Said fectiefs- 3,041,3946/1962 schrecongost 17a-7.3 6. An automatic frequency controlarrangement for television receivers according to claim 5 wherein 5FOREIGN PATENTS the parallel combination of said first rectifier andsaid rst resistor is connected in series relation with said capacitiveload circuit across said first source and the parallel combination ofsaid second rectifier and ROBERT L GRIFFIN Primary Examiner said secondresistor is connected directly across said 10 R L RICHARDSONs AssistantExaminer first source.

1,255,296 1/1961 France.

References Cited U.S Cl. X.R

The following references, cited by the Examiner, are of record in thepatented file of this patent or the original patent.

